As of Elgg 1.8, the css is split into several files based on what
aspects of the site you’re theming. This allows you to tackle them one
at a time, giving you a chance to make real progress without getting
overwhelmed.

Here is a list of the existing CSS views:

elements/buttons.css: Provides a way to style all the different kinds of buttons your site will use. There are 5 kinds of buttons that plugins will expect to be available: action, cancel, delete, submit, and special.

elements/chrome.css: This file has some miscellaneous look-and-feel classes.

elements/components.css: This file contains many “css objects” that are used all over the site: media block, list, gallery, table, owner block, system messages, river, tags, photo, and comments.

elements/forms.css: This file determines what your forms and input elements will look like.

elements/icons.css: Contains styles for the icons and avatars used on your site.

elements/modules.css: Lots of content in Elgg is displayed in boxes with a title and a content body. We called these modules. There are a few kinds: info, aside, featured, dropdown, popup, widget. Widget styles are included in this file too, since they are a subset of modules.

elements/navigation.css: This file determines what all your menus will look like.

elements/typography.css: This file determines what the content and headings of your site will look like.

rtl.css: Custom rules for users viewing your site in a right-to-left language.

admin.css: A completely separate theme for the admin area (usually not overridden).

elgg.css: Compiles all the core elements/* files into one file (DO NOT OVERRIDE).

elements/core.css: Contains base styles for the more complicated “css objects”. If you find yourself wanting to override this, you probably need to report a bug to Elgg core instead (DO NOT OVERRIDE).

elements/reset.css: Contains a reset stylesheet that forces elements to have the same default

Starting in Elgg 1.8, we’ve provided you with some development tools to help you
with theming: Turn on the “Developers” plugin and go to the “Theme
Preview” page to start tracking your theme’s progress.